Modern Post-Frame Structural Design Practices: An Introduction

Transcription

1 Modern Post-Frame Structural Design Practices: An Introduction Presented on March 4, 2015 by: Harvey B. Manbeck, PhD, PE Consultant to NFBA Professor Emeritus, Engineering Penn State University Disclaimer: This presentation was developed by a third party and is not funded by WoodWorks or the Softwood Lumber Board.

2 The Wood Products Council is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES), Provider #G516. Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-aia members are available upon request. This course is registered with AIA CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner ofhandling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

3 Course Description This session is intended for architects and designers who want to understand basic structural design methods for an engineered post-frame building system. Without delving into engineering details or calculation procedures, it covers the components of the post-frame building system, as well as key structural design concepts. Two approaches are highlighted in particular: one for post-frame systems without diaphragm action, the other for post-frame systems with diaphragm action. Procedures for designing isolated pier foundations for post-frame buildings are also discussed, as are technical resources available to the design professional.

4 Learning Objectives 1. Identify the primary structural components of post frame (PF) building systems 2. Learn basic procedures for conducting structural analysis of PF systems with and without diaphragm action 3. Define the design approach for isolated post/pier PF foundations 4. Recognize post-frame design resources available to architects and engineers

5 LEARNING OBJECTIVES Identify the primary structural components of post-frame (PF) building systems Learn basic procedures for conducting structural analysis of PF systems with and without diaphragm action Define the design approach for isolated post/pier PF foundations Recognize post-frame design resources available to architects and engineers

6 POST-FRAME (PF) BUILDING Wood industry s counterpart to low profile (1 to 2-1/2 story) steel buildings Developed in late 1930 s for agricultural sector Known as pole building in the past PF has evolved to highly engineered wood building system PF has expanded to many commercial, residential & institutional applications SYSTEMS

7 TYPICAL PF BUILDING SYSTEM Sheathing: 26 to 29 ga Ribbed Steel OR OSB or Plywood Roof Purlins Typ. 2x4s on edge or flat Roof Framing Trusses or Rafters Wall Girts Typ. 2x4 or 2x6 flat Laminated or Solid- Sawn Wood Columns

8 PF BUILDING SYSTEM FOUNDATION OPTIONS 9 Continuous RC Foundation Wall Isolated Pier Foundation Thickened Edge of Concrete Slab

9 PRIMARY PF DESIGN METHODS 2-dimensional frame design method Without diaphragm action 3-dimensional diaphragm design method With diaphragm action

10 PF SYSTEMS WITHOUT DIAPHRAGM ACTION Unsheathed walls Unsheathed walls

11 PF SYSTEM WITH DIAPHRAGM ACTION Sheathed Version of This Building

12 LATERAL LOADS: WITHOUT DIAPHRAGM ACTION Wind Wind direction Typical sway (Δ) of interior post frame at design lateral load = 5 to 8 inches

13 LATERAL LOADS: WITH DIAPHRAGM Typical sway ( 1 ) of centermost post-frame at design lateral load = 0.5 to 1.0 inch ACTION Wind direction 1

14 ADVANTAGES OF DIAPHRAGM DESIGN Smaller sidewall posts Shallower post or pier embedment depths Benefits: More economical design Greater structural integrity More durable post-frame structures

15 FULL-SCALE PF BUILDING TESTS 29 ga ribbed steel sheathing Load cell & Displacement Transducer Hydraulic cylinder 16 ft 5 ft 40 ft W x 80 ft L x 16 ft H (8 ft. o.c.)

16 DIAPHRAGM VS NO DIAPHRAGM ACTION

17 WHEN TO USE 2-D FRAME DESIGN METHOD Side or endwalls are open, or not sheathed PF Building with L:W 2.5 to 3:1 Connections and other structural detailing don t develop a continuous load path for transfer of in-plane shear forces Through the roof sheathing Between the diaphragm and the top of the endwall Through the endwall or shearwall Between bottom of the endwall and the endwall foundation

18 EMBEDDED POST/PIER FOUNDATIONS Common post-soil fixity models for embedded post or pier foundations: Constrained post or pier Non-constrained post or pier

19 POST/PIER EMBEDMENT DESIGN Horizontal movement permitted Horizontal movement prevented by floor or mechanical connection d 0 Non-constrained Constrained

20 POST FOUNDATIONS-Simplified Model: NON-CONSTRAINED CASE Load Direction Structural Rotation Analog for Determining Point Post Ground Surface Shear (V G ) and Moment (M G ) d w w V G M G d Fixed end at depth Slab w below grade w = face width of post bearing against soil Non-constrained post/pier Constrained post/pier

21 POST FOUNDATIONS-Simplified Model: CONSTRAINED CASE Load Direction Structural Analog for Determining Post Ground Surface Shear (V G ) and Moment (M G ) Vertical roller at top edge d of slab w d Rotation Point Slab V G M G Fixed end at ground line w Non-constrained post/pier Constrained post/pier

22 PRIMARY ASSUMPTIONS FOR THE SIMPLIFIED MODEL Soil is homogeneous throughout the entire embedment depth. Soil stiffness is either constant (cohesive soils) for all depths below grade or linearly increases (non-cohesive soils) with depth below grade. Width of the below-grade portion of the foundation is constant. This generally means that there are no attached collars or footings that are effective in resisting lateral soil forces.

23 UNIVERSAL MODEL POST FOUNDATIONS Used to determine ground surface shear, V G, and moment, M G when required conditions for simplified method not met Considers the load-deformation behavior of the soil surrounding the embedded post Soil foundation load deformation behavior evaluated using soil spring models

24 UNIVERSAL MODEL: SOIL LOAD - DISPLACEMENT BEHAVIOR Ultimate Soil Strength, p u,z (psi) Elastic-Perfectly Plastic Soil Soil Load (psi) Slope = soil stiffness, E s (lb/in) Soil Deformation (in.)

25 POST FOUNDATIONS-Universal Model: NON-CONSTRAINED CASE M V M U V U t F ult,1 Point of foundation rotation d RU z t 2 t 3 t 4 t d RU 2 3a 3b 4 5 F ult,3b F ult,4 F ult,5 F ult,2 F ult,3a

26 POST FOUNDATIONS-Universal Model: Post contacts ground surface restraint CONSTRAINED CASE M U V U t 1 1 F ult,1 t 2 2 F ult,2 z t 3 3 F ult,3 t 4 4 F ult,4 t 5 5 F ult,5

27 DESIGN METHODS: 2-D POST FRAME s x w Wind Direction s x q wr s x q lr Each frame is designed to carry its full tributary lateral and gravity loads H 2 s x q ww s x q lw Post-to-truss connections usually modeled as a pin H 1 W The post-to-ground reaction is modeled consistent with post embedment details. (Note that one post foundation may be constrained and the other nonconstrained)

28 ASCE-7 Governing Load Combinations (ASD) Dead + ¾ snow + ¾ wind (or seismic) or 0.6 dead + wind (or seismic) Usually controls post design 2-D DESIGN ANALYSIS Dead + snow (balanced & unbalanced) Usually controls roof-framing design

29 SIMPLIFIED 2-D PF DESIGN METHOD V = roof truss vertical reaction Wind direction P = ½ (Resultant lateral roof load from truss) ½ (q ww +q lw ) x s or Max(q ww, q lw ) x s Specify dead & snow loads for truss manufacturer Then design the post for the design lateral load combinations Model post-to-soil interaction appropriately (For constrained pier foundation and simplified method, this is fixed end at ground line.)

30 DIAPHRAGM DESIGN METHOD Incorporates in-plane shear strength and stiffness of the roof and wall sheathing to transfer design lateral loads to the foundation Three-dimensional structural analysis method Significantly decreases wall-post size and postfoundation embedment depth Will use an on-line structural analysis program, DAFI

31 PF DIAPHRAGM DESIGN Key Definitions - In-plane shear stiffness of the roof diaphragm panel, c - Bare frame stiffness of the post-frame, k - Design eave lateral load, P

32 DIAPHRAGM TEST PANEL b sp = Slope length (roof diaphragm length) Test panel width, a Test panel length, b Roof span Endwall θ Test panel (basic element) a p Roof sheet end joint Building length = L B Building width

33 DIAPHRAGM TEST PANEL Purlin (chord) Sheathing/ cladding Rafter or truss top chord (strut)

34 CANTILEVER TEST CONFIGURATION P = applied force Truss top chord Purlin b = Test diaphragm length Cladding s a = Test diaphragm width Direction of corrugations

35 DIAPHRAGM TEST RESULTS, IN- PLANE STRENGTH & STIFFNESS P Diaphragm Test Panel Schematic Ultimate Strength = P ult P Design shear strength = 0.4 P ult Design unit shear strength = (1/b)0.4 P ult 1 c 1 C = design shear stiffness (slope)

36 DIAPHRAGM TEST PANEL b sp = Slope length (roof diaphragm length) Test panel width, a Test panel length, b Roof span Endwall θ Test panel (basic element) a p Roof sheet end joint Test panel shear props from sheathing supplier or from PFBDM Building width Roof diaphragm shear props deduced from test panel props

37 DIAPHRAGM DESIGN METHOD Shear stiffness of a roof diaphragm panel test panel stiffness, c roof panel width, a p ROOF PANEL STIFFNESS roof panel roof slope length b sp roof slope Θ c h = [c (a/b)] (b sp /a p )cos 2 Θ

38 DIAPHRAGM DESIGN METHOD-ROOF PANEL STRENGTH In-plane strength is a linear function of diaphragm length, b sp V = [unit shear strength](roof diaphragm length) V = [0.4(P ult /b)](b sp )

39 Pinned Connection DIAPHRAGM DESIGN METHOD- BARE FRAME STIFFNESS, K P1 Model soil to post interaction using appropriate structural analog for constrained or non-constrained pier

40 PF diaphragm design procedures based on: 1. compatibility of postframe and roof panel eave deformations and 2. Equilibrium of horizontal forces at each eave DIAPHRAGM DESIGN METHOD P = Design Lateral Eave Load

41 DIAPHRAGM DESIGN METHOD Equilibrium of forces at each PF eave P i = P fi + P ri P i = design eave load in i th PF P fi = portion of the design eave load carried by the i th PF P ri = portion of the design eave load carried by the roof diaphragm panel at the i th PF

42 Compatibility of roof and PF deformations at each PF eave Δ ri = Δ fi Δ ri = roof panel eave deformation at the i th PF (dependent upon c i, k i, and P i ) Δ fi = P fi /k i DIAPHRAGM DESIGN METHOD

43 DAFI COMPUTER PROGRAM DAFI program calculates Eave displacement of each post frame Portion of the design eave load carried by each post frame Shear forces carried by each roof diaphragm panel in the building system Available at no cost at

44 Total number of bays in the building DAFI INPUTS Design eave loads at each post frame, P i Bare frame stiffness of each post frame, k i In-plane shear stiffness of each roof diaphragm panel, c hi

45 DIAPHRAGM DESIGN METHOD

46 DIAPHRAGM DESIGN STRUCTURAL ANALOG Panel/PF structural analog of a 3-bay building PF 1 (k 1 ) 1 2 (k 2 ) (k 3 ) 3 (k 4 ) 4 Diaphragm Panel 1(c h1 ) 2(c h2 ) 3(c h3 ) P1 P2 P3 P4

47 DAFI: UNDEFORMED POSITION Node Datum Datum

48 DAFI: DEFORMED EQUILIBRIUM POSITION Datum Datum

49 DAFI COMPUTER PROGRAM P f1 P f2 P f3 P f4

50 DAFI COMPUTER PROGRAM V 2 V 1 V 3

51 DAFI: HIGHLY FLEXIBLE Can be used for post-frame building systems where: Stiffness, k i, of the interior post frame elements are not the same Stiffness, c hi, of the diaphragm panel elements are not the same Stiffness, k i of the two endwall post-frames are not the same Available at no cost to designers at

52 DAFI: MINI DEMONSTRATION 48-ft-wide by 96-ft-long post frame Post frames 8-ft o.c. Number of bays 12 Post-frame stiffness (k) 300 lbs/in. Endwall stiffness (k e ) 10,000 lbs/in. Roof diaphragm stiffness (C) 12,000 lbs/in. Horizontal eave load at interior post frame 800 lbs

53 DAFI: MINI DEMONSTRATION Access DAFI by: Going to Clicking onto DAFI icon Running DAFI when prompted NOTE: Recommend you use Explorer or Firefox browser

54 DAFI: MINI DEMONSTRATION

55 DAFI: MINI DEMONSTRATION

56 DAFI: MINI DEMONSTRATION

57 DAFI: MINI DEMONSTRATION

58 POST/PIER EMBEDMENT DESIGN Load Direction Rotation Point d w d Slab w Non-constrained post/pier Lateral displacement Allowed at ground surface Constrained post/pier Lateral displacement = zero At ground surface

59 POST/PIER EMBEDMENT DESIGN Post-embedment details must resist Downward acting gravity loads Shear force and moments from lateral loadings Uplift post loads ANSI/ASAE EP486.2, Shallow post and pier foundation design

60 POST/PIER EMBEDMENT DESIGN: Two Design Approaches LATERAL LOADS Simplified Method Universal Method

61 POST/PIER EMBEDMENT DESIGN: LATERAL LOADS-SIMPLIFIED METHOD Constrained at Groundline Ground surface P M Gu V u G R Ground surface P U M U V U M U M 3 b S U 3 b S U U R Ground surface P U V U R y Restraint Restraint y Restraint y d z Soil forces 3 d b K P P Case 1 Cohesionless Soil Post or pier with width b Soil with a fixed modulus of horizontal subgrade reaction k C d 3 b S U d S U P U z Post or pier with width b and depth d < 4b Cohesive soil with an undrained shear strength S U Case 2 Cohesive Soil d 4 b 9 b S U P U z Post or pier with width b Cohesive soil with an undrained shear strength S U (a) d 4b (b) d > 4b

62 POST/PIER EMBEDMENT DESIGN: LATERAL LOADS-SIMPLIFIED METHOD Constrained at Ground Surface Design Criteria CASE 1: (Cohesionless Soil) M u = d 3 bk p γ Design Ultimate Moment Capacity (M G *f L ) M u = ultimate groundline moment capacity d = embedment depth b = foundation width bearing against soil γ = soil density K p = passive pressure coefficient (1 + sinϕ)/(1 sinϕ) M G = Calculated ground surface post moment f L = ASD factor of safety

63 POST/PIER EMBEDMENT DESIGN: LATERAL LOADS-SIMPLIFIED METHOD Constrained at Ground Surface Design Equation CASE 2: (Cohesive Soil) (a) d 4b M u = d 3 bs u [3/2 + d/(2b)] M G (f L ) (b) d > 4b M u = bs u (4.5d 2 16b 2 ) M G (f L ) where S u = Soil undrained shear strength (soil cohesion)

64 POST/PIER EMBEDMENT DESIGN: LATERAL LOADS-SIMPLIFIED METHOD Non-Constrained at Ground Surface P U P U P U Ground surface d d RU z y Post or pier with width b Point of rotation 3 d b K P P U M U V U 3 d RU b K P Case 1 Cohesionless Soil Cohesionless soil with density and friction angle Ground surface Post/pier with width b and d RU < 4b Cohesive soil with undrained shear strength S U Point of rotation 9 b S U P U M U V U z 3 b S U y 3 b S U d RU S U d RU d Ground surface Post/pier with width b and d RU > 4b Cohesive soil with undrained shear strength S U Point of rotation 9 b S U See ANSI/ASAE EP486.2 or PFBDM for Design Equations (a) d Ru 4b and d 4b Case 2 Cohesive Soil P U M U V U 9 b S U 3 b S U 4 b z d RU (b) d Ru > 4b and d > 4b y d

65 POST/PIER EMBEDMENT LATERAL LOADS- UNIVERSAL METHOD Constrained at ground surface Spring Model Post contacts ground surface restraint M U V U t 1 1 F ult,1 z t 2 t 3 t F ult,2 F ult,3 F ult,4 Foundation moment and shear capacity from basic mechanics t 5 5 F ult,5

66 POST/PIER EMBEDMENT LATERAL LOADS- UNIVERSAL METHOD Non-constrained at ground surface Design Criteria Foundation moment and shear capacity from basic mechanics

67 POST/PIER EMBEDMENT LATERAL LOADS- UNIVERSAL METHOD Design Articles in Frame Building News Bohnhoff, David Modeling Soil Behavior with Simple Springs, Part 1: Spring Placement and Properties. Pages 49 to 54. April. Bohnhoff, David Modeling Soil Behavior with Simple Springs, Part 2: Determining the Ultimate Lateral Capacity of a Post/Pier Foundation. Pages 50 to 55. June.

68 POST/PIER EMBEDMENT DESIGN: UPLIFT RESISTANCE Mass of soil in shaded zone resists post withdrawal due to uplift forces Post must be mechanically attached to the collar or wood cleat Mass of attached collar or wood cleat B u

69 Governing Design Equations POST/PIER FOUNDATION EMBEDMENT UPLIFT LOADS Weight of attached collar/footing + Weight of soil above attached collar/footing Design uplift load of post frame

70 POST/PIER FOUNDATION EMBEDMENT UPLIFT LOADS, U Shallow vs. Deep Foundations Deep foundation (d U h) Shallow foundation Shallow: d u h d U Failure plane h d U Deep: d u h B U Uplift resistance (h dependent upon soil internal angle of friction)

71 Ultimate uplift resistance of soil above circular anchorage systems Cohesive Soils: U = γd u (B u2 π/4-a p ) + F c S u B u2 π/4 d u = post embedment depth γ = soil density POST/PIER FOUNDATION EMBEDMENT UPLIFT LOADS B u = anchor diameter A p = post cross sectional area F c = breakout factor for soil uplift (1.2d u /B u )

72 POST/PIER FOUNDATION EMBEDMENT UPLIFT LOADS U-value equations provided in ASAE/ANSI EP and PFBDM for additional cases Cohesive soils rectangular uplift anchors Cohesionless soils circular uplift anchors (shallow and deep foundations) Cohesionless soils rectangular uplift anchors (shallow and deep foundations)

73 POST/PIER FOUNDATION DESIGN: UPLIFT DESIGN Design Equations for Uplift Resistance of Embedded Posts with Uplift Anchors 1. Post Frame Building Design Manual (2014 ed.) ( or 2. ANSI/ASAE EP486.2, Shallow Post & Pier Foundation Design (

74 ANSI/ASAE (ASABE) EP 484 Diaphragm design procedures ANSI/ASAE (ASABE) EP Shallow post & pier foundation design ANSI/ASAE (ASABE) EP 559 Requirements and bending properties for mechanically laminated columns asabe.org or nfba.org POST-FRAME TECHNICAL RESOURCES

75 POST-FRAME TECHNICAL RESOURCES Provides structural design procedures, commentary & design examples for postframe building systems

76 OTHER PF TECHNICAL RESOURCES Post Frame Construction Guide Post Frame Construction Tolerance Guidelines

77 OTHER PF TECHNICAL RESOURCES DAFI or

78 MORE PF DESIGN GUIDANCE? National Frame Building Association 8735 Higgins Road Suite 3000 Chicago, IL 60631

79 Questions? This concludes The American Institute of Architects Continuing Education Systems Course Harvey B. Manbeck, P.E., PhD National Frame Building Assn.